Spectroscopy systems and methods using quantum cascade laser arrays with lenses

ABSTRACT

A spectroscopy system includes an array of quantum cascade lasers (QCLs) that emits an array of non-coincident laser beams. A lens array coupled to the QCL array substantially collimates the laser beams, which propagate along parallel optical axes towards a sample. The beams remain substantially collimated over the lens array&#39;s working distance, but may diverge when propagating over longer distances. The collimated, parallel beams may be directed to/through the sample, which may be within a sample cell, flow cell, multipass spectroscopic absorption cell, or other suitable holder. Alternatively, the beams may be focused to a point on, near, or within the target using a telescope or other suitable optical element(s). When focused, however, the beams remain non-coincident; they simply intersect at the focal point. The target transmits, reflects, and/or scatters this incident light to a detector, which transduces the detected radiation into an electrical signal representative of the target&#39;s absorption or emission spectrum.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit, under 35 U.S.C. §119(e), of U.S.Application No. 61/640,482, filed Apr. 30, 2012, and entitled“Spectroscopy Systems and Methods Using Quantum Cascade Laser Arrayswith Lenses,” which is incorporated herein by reference in its entirety.

BACKGROUND

An individual tunable semiconductor laser can be used for spectroscopicanalysis of solids, gases and liquids. Sometimes, the laser is opticallycoupled to an absorption cell, which serves two roles in thespectrometer system: (1) the absorption cell provides a definedabsorption path length so that quantitative analysis of the sample canbe implemented according to, for example, the Beer-Lambert relation formolecular absorption; and (2) the absorption cell can be configured toallow for greater optical path length than any single cell dimensionthrough the use of multipass geometries. Greater optical path lengthtranslates to greater absorption signal, which in turn translates into ahigher Signal to Noise Ratio (SNR) and a higher sensitivity.

Conventionally, when a tunable laser is used with an optical absorptioncell, light from the laser is substantially collimated into the cell soas to reduce divergence and allow greater transmission through the celland higher detection efficiency. This is true whether the light makesone single pass through the cell before detection or whether ittraverses more than one pass as in Herriott cell or White cell basedspectroscopy, cavity ringdown spectroscopy, and integrated cavity outputspectroscopy (collectively: “multipass spectroscopy”). For more onintegrated cavity output spectroscopy, see U.S. Pat. No. 6,795,190,entitled “Absorption Spectroscopy Instrument with Off-Axis LightInsertion into Cavity,” which is incorporated herein by reference in itsentirety.

Not all spectroscopy systems use absorption cells. For example, thelaser beam may be directed toward a target and its wavelength tunedwhile the backscattered photons are detected and analyzed. A commoninstance of this embodiment is the standoff detection of surfaceadsorbed condensed phase material using, for example, infraredabsorption or Raman spectroscopies.

For broader wavelength coverage tunable laser arrays can also be usedfor spectroscopy, both with and without absorption cells. For more oncontinuously or broadly tunable laser arrays, see U.S. Pat. No.7,826,509 “Broadly Tunable Single-Mode Quantum Cascade Laser Sources andSensors,” which is incorporated herein by reference in its entirety. Inthis work, an array of single frequency Quantum Cascade Lasers, eachmember with its own wavelength, is used to obtain broader absorptionspectra than would be obtainable using just one single frequency laser.This is especially useful for the spectroscopic analysis of condensedphased materials and for analysis of multiple gases, where broaderspectral coverage is highly beneficial.

In either case, using a single emitter or an array of emitters, thelaser beams' divergence often limits the obtainable signal to noiseratio as high divergence reduces the available optical transmitted orbackscattered optical power per unit area for detection wheninterrogating samples over long path lengths, including those inmultipass sample cells or standoff spectroscopy. In addition, portionsof the diverging beams may interrogate different portions of the samplevolume. As a result, different portions of the detected absorptionspectrum may correspond to different parts of the sample. This can beespecially troublesome when interrogating condensed phase samples thatare not perfectly homogenous.

In the case of single emitters, the common solution to reduce divergenceis to place an optical element with focusing power, such as a lens, infront of the emitter. However, for emitter arrays, a single lens is notalways appropriate. This is illustrated in FIG. 1, which illustrates howdiverging beams 111 emitted by lasers 112 in a laser array 110 arecoupled to a single lens 120. The lens 120 is positioned with itsoptical axis perpendicular to the face of the laser array 120 at adistance roughly equal to the lens's focal length and produces a set ofcollimated beams 121. Although the lens 120 collimates the divergingbeams 111 emitted by the laser array, the resulting collimated beams 121are tilted with respect to each other by an amount that depends on boththe beam's position. If the lens 120 has a focal length ƒ, thecollimated beam 121 from the nth laser in the array 110 points at anangle given by:

$\begin{matrix}{{\Delta\;\theta_{n}} = {\tan^{- 1}\left( \frac{\Delta\; x_{n}}{f} \right)}} & (1)\end{matrix}$In equation (1), Δθ_(n) is measured with respect to the axis of the lens120 and Δx_(n) is the transverse position of the nth laser in the array110 relative to the focal point of the lens 120. In this case, the laserbeams are spatially separated in the far-field 130, as shown. Thus,collimation of diverging beams 111 from an entire array 110 using asingle lens 120 exacerbates, rather than corrects, many of the problemsassociated with using a tunable laser array instead of a single tunablelaser for spectroscopy by introducing an angular difference betweenoutput beams where previously there was only a lateral offset.

Wavelength beam-combining (WBC) (also known as spectral beam combining)is a technique used in telecommunications and spectroscopy to addressthe problem of position- and wavelength-dependent steering caused bycollimating beams from an array. In WBC, the laser beams from an arrayare merged into a single, co-linear beam of spatially overlappingoutputs that can then be propagated into the far field or perhapsthrough an absorption cell. This method takes on many physicalembodiments, depending on the application.

For example, in Wavelength Division Multiplexing, a central technique intelecommunications, the outputs of multiple fiber-coupled singlefrequency lasers are merged into one single fiber for long distancetransmission.

Yet another example is the WBC of free space infrared lasers usingeither “open loop” or “closed loop” approaches. One example of open loopbeam combining involves an array of single frequency QCLs that arecoupled to a non-actively aligned dispersion element using either anarray of matched lenses or a single collection lens. The blaze spacingand angle of the grating are chosen to correct for the physical andwavelength separation of the array members such that the gratingdisperses each laser wavelength at a slightly different angle. Theresult is that all wavelengths are overlapped in space and propagatedtogether (co-linearly) in the far field.

In the closed loop approach to WBC, the array instead comprisesmore-or-less identical emitters, where the emission wavelength is notdifferentiated in the laser itself. Rather, the dispersion element, incombination with an output coupler, acts to form a laser cavity suchthat each laser's frequency is determined by feedback from thedispersion element. As with the “open loop” approach, this techniqueresults in a beam of spatially overlapping contributions of differentfrequencies.

In both open-loop and closed-loop WBC, a beam-combined laser arraysystem has some advantages over non-beam combined array outputs,including higher degree of spatial symmetry and an improved beam qualityparameter, M². Indeed, beam combining of QCL arrays has be shown toproduce near diffraction limited (where M²=1) performance in thefar-field. Improved beam symmetry, spatial overlap, and M² are alluseful in many applications in molecular spectroscopy, infraredcountermeasures (IRCM), and fiber coupling of laser arrays.

SUMMARY

Aspects and embodiments are directed to methods and apparatus that applylaser arrays with lens arrays and a detector for spectroscopicinterrogation. As discussed in detail below, the output of an array oftunable lasers may be collected with an array of lenses or microlensesand used for detection and quantification of chemicals/compounds thathave absorption features within the spectral range of the emitter array.

Embodiments of the present invention include systems and methods forsensing a spectroscopic signature of a sample, such as a bodily fluid, agas, an adsorbed solid, an adsorbed liquid, a gas phase component of asurface-adsorbed solid, or a gas phase component of a surface-adsorbedliquid, without WBC or other beam-combining techniques. In oneembodiment, the system comprises a transmitter, which includes an arrayof quantum cascade lasers (QCLs) and an array of lenses, and atransmitter. In operation, the array of QCLs emits an array ofnon-coincident (e.g., substantially parallel or intersecting) laserbeams to the array of lenses, which substantially collimates the arrayof non-coincident laser beams. The transmitter transmits this array ofnon-coincident laser beams to the sample (e.g., over a distance of 10cm, 1 m, 5 m, 10 m, or 100 m) so as to produce radiation that isscattered, reflected, and/or transmitted by the sample. The receiver,which is also in optical communication with the sample, detects at leasta portion of the radiation and provides a signal representative of thespectroscopic signature of the sample based on the detected radiation.

In some examples, the array of QCLs is configured to tune a wavelengthof one or more laser beams in the array of laser beams without changingthe wavelength-tuned beams' far-field pointing angles. (Conversely,tuning the wavelength of a beam in a WBC system causes the beam'sfar-field pointing angle to change as explained below.) The array ofQCLs may also be configured to generate one or more of the laser beamsin the array of non-coincident laser beams at different wavelengths,e.g., so as to provide a frequency comb or an array of laser beams witha predetermined wavelength spacing.

Some examples of the array of lenses include at least one lens having abeam divergence angle of about 0.5 degrees to about 8.0 degrees (e.g.,about 5.0 degrees). The array of lenses may also include at least oneaspheric lens, spherical lens, diffractive lens, or graded-index lens.The transmitter may also include a telescope, in optical communicationwith the array of lenses, to cause the array of non-coincident laserbeams to propagate towards the sample. In some geometries, the telescopemay also collect radiation that is scattered and/or reflected from thesample.

Exemplary systems may also include a sample holder to hold the sample inthe path of the array of non-coincident laser beams. For instance, thesampler holder may include a transmissive cell, an adsorbing surface, amicrofluidic channel, and/or a multipass absorption cell, which may

For solid or liquid samples, the sample holder may hold the sample suchthat the array of non-coincident laser beams strikes the sample'ssurface. Liquid and gaseous samples may flow through the sample cellduring illumination or between bursts of illumination from thetransmitter.

In another embodiment, a system for sensing a spectroscopic signature ofa sample includes an array of QCLs, an array of lenses, a multipassabsorption cell, and a detector. In operation, the array of QCLs emitsan array of substantially parallel laser beams comprising lasers beamsat different wavelengths. The array of lenses, which is in opticalcommunication with the array of QCLs, substantially collimates the arrayof parallel laser beams. The multipass absorption cell, which is inoptical communication with the array of lenses, receives the array ofparallel laser beams from the array of lenses, directs the array ofparallel laser beams along multiple passes (e.g., 10, 20, 30, 40, 50, or100 passes) through a least a portion of the sample, and emits the arrayof parallel laser beams after it propagates through the sample. Thedetector, which is in optical communication with the multipassabsorption cell, detects the array of parallel laser beams so as toprovide a signal representative of the spectroscopic signature of thesample.

In some examples, the system also includes a current source and/or aheatsink. The current source is in electrical communication with thearray of QCLs, and the heatsink is in thermal communication with thearray of QCLs. The current source and/or the heatsink may be used totune a wavelength of one or more laser beams in the array of laser beamswithout changing the wavelength-tuned laser beams' far-field pointingangle(s).

Other aspects, embodiments, and advantages of these exemplary aspectsand embodiments, are discussed in detail below. Any embodiment disclosedherein may be combined with any other embodiment in any mannerconsistent with at least one of the objects, aims, and needs disclosedherein, and references to “an embodiment,” “some embodiments,” “analternate embodiment,” “various embodiments,” “one embodiment” or thelike are not necessarily mutually exclusive and are intended to indicatethat a particular feature, structure, or characteristic described inconnection with the embodiment may be included in at least oneembodiment. The appearances of such terms herein are not necessarily allreferring to the same embodiment. The accompanying drawings are includedto provide illustration and a further understanding of the variousaspects and embodiments, and are incorporated in and constitute a partof this specification. The drawings, together with the remainder of thespecification, serve to explain principles and operations of thedescribed and claimed aspects and embodiments.

BRIEF DESCRIPTION OF DRAWINGS

Various aspects of at least one embodiment are discussed below withreference to the accompanying figures, which are not intended to bedrawn to scale. Where technical features in the figures, detaileddescription or any claim are followed by references signs, the referencesigns have been included for the sole purpose of increasing theintelligibility of the figures, detailed description, and claims.Accordingly, neither the reference signs nor their absence are intendedto have any limiting effect on the scope of any claim elements. In thefigures, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in every figure.The figures are provided for the purposes of illustration andexplanation and are not intended as a definition of the limits of theinvention. In the figures:

FIG. 1 is a ray trace simulation of a closely spaced 1-D laser arraycoupled to a single spherical collimation lens placed one focal lengtharray from the center laser.

FIG. 2 is a ray trace of the same laser array shown in FIG. 1B coupledto a monolithic array of lenses.

FIG. 3 is a schematic diagram of an absorption spectroscopy system thatincludes the microlens-coupled laser array shown in FIG. 2.

FIG. 4 is a schematic diagram of a single collimated laser coupled to aHerriott cell for analysis of gases using absorption spectroscopysystem.

FIG. 5 is a schematic diagram of the laser beams from the microlenscoupled laser array shown in FIG. 2 being launched to the same Herriottcell as in FIG. 4.

FIG. 6 is a schematic diagram of the laser beams from the microlensedcoupled laser array shown in FIG. 2 being launched into the far fieldthrough a refractive telescope, which also serves to collect light thatis scattered from a target.

FIG. 7 is a schematic diagram of the laser beams from the microlensedcoupled laser array shown in FIG. 2 being launched into the far fieldthrough a reflective telescope, which also serves to collect light thatis scattered from a target.

FIG. 8 shows the beam profile of a lensed array of five lasers forvarious distances from the array.

FIG. 9 shows an apparatus for examining blood glucose in human bloodusing a QCL array and detector.

FIG. 10 shows a contour model of a microlens array where only the curvedsurface is included for clarity. This particular array is a 1×15 elementsystem where 1 is the number of lenses in the Y direction and 15 is thenumber of lenses in X.

FIG. 11 shows a three-dimensional design for a packaged QCL array whichincludes an array of microlenses, thermoelectric cooler, electricalcontacts, enclosure, and exit window.

DETAILED DESCRIPTION

FIG. 1 illustrates an optical system in which an array 110 ofone-dimensional Quantum Cascade Lasers 112 (two-dimensional laser arraysare also possible). As understood by those of skill in the art, QCLs 112are electrically driven semiconductor lasers. They may be efficient,reliable, and compact enough to be separated from each other by about 20μm to about 500 μm. QCLs 112 emit light or lase at room temperature atwavelengths spanning about 3 μm to about 24 μm. They may also emitradiation at Terahertz frequencies. These wavelength ranges overlap manymolecular absorption lines of interest. The QCLs 112 in the array 110may employ distributed feedback (DFB) or any other suitable mechanism topromote emission at a single longitudinal frequency. Each QCL 112 in thearray 110 may lase at a unique frequency with a tuning range that iscontiguous with or overlaps the tuning range of one or more other QCLs112 in the array 110. Individual QCLs 112 can achieve watt-level outputpower in continuous-wave operation at room temperature and can bedesigned to have broadband gain such that the lasing wavelength can betuned over a broad spectral range of approximately 300 cm⁻¹. The QCLs112 in the array 110 may be operated in any number of temporalpermutations, from one at a time to fully simultaneously.

In the case of FIG. 1, the emission 111 from the array 110 is directedonto a refractive collimating lens 120 to produce a slightly focused 121but poorly collimated beam 130. Such a beam 130 is of limited utilityfor chemical sensing and other applications due to the high divergence,which reduces the power density dramatically and nonlinearly.

As described above (and in U.S. Patent Application Publication No.2012/0033697 to Goyal et al., which is incorporated herein by referencein its entirety), wavelength beam combining (WBC) can be used totransform a set of beams from a laser array into single beam. For manyapplications, however, WBC is not necessary and may even be undesirabledue the added complexity and cost of introducing and aligning a grating.While WBC also improves beam symmetry, this too is often unnecessary inone- and two-dimensional tunable laser arrays.

Because the QCL array spacing (which may be, e.g., 20-500 μm) may bemuch smaller than the beam diameter normally employed in spectroscopicapplications (which may be, e.g., 5-50 mm), further beam combining suchas WBC is not always necessary or desired in spectroscopic systems. Inaddition, WBC with a diffractive element suffers from beam steering asthe laser wavelength is tuned due to the change in diffraction angle asthe wavelength changes. However, using lens arrays in combination withtraditional reflective or refractive elements eliminates this problem.

FIG. 2 illustrates a spectroscopy system 200 that employs aone-dimensional array 210 of quantum cascade lasers (QCLs) 212.(Two-dimensional QCL arrays are also possible.) This spectroscopy system200 can be used to analyze liquids and gases in multiple samplingconfigurations, including those disclosed herein. The spectroscopysystem 200 also includes an array 220 of lenses 222, or microlenses 222,where each lens 222 in the array 220 has a focal length ƒ, which mayrange from hundreds of microns to tens of centimeters. The lenses 222 inthe lens array 220 may be spherical lenses, aspheric lens, astigmaticlenses, cylindrical lenses, graded index (GRIN) lenses, diffractivelenses, refractive/diffractive lenses, or any other suitable type oflens. The lens array 220 may be monolithic and can be affixed to orintegrated with laser array 210 once properly aligned. Alternatively,the system 200 may include an adjustment tool to align and/or positionthe lens array 220 with respect to the laser array 210.

As shown in FIG. 2, the lens array 220 is disposed a distance ƒ from thelaser array 210 such that the lens array 220 substantially collimatesthe diverging beams 211 emitted by the laser array 210. The resultingsubstantially collimated beams 221 propagate along parallel opticalaxes. Each individual beam in the figure has a diameter of about 0.2 mmto about 3.0 mm, depending on the focal length of the lenses used aswell as on lens clear aperture. Similarly, each beam is of high Gaussiancharacter such that the beam quality parameter M² of each laserscollimated output ranges between about 1.0 and about 1.7. The lens array220 may also be moved closer to or farther from the laser array 210 toproduce beams that come to a focus or diverge as desired. In any case,the beams transmitted through the lens array remain non-coincident—thatis, they propagate along optical axes that are parallel, intersecting,or skew rather than overlapping.

The parallel collimated beams 221 propagate to a sample holder 230 thatcontains a sample 231 for spectroscopic interrogation. The sample holder230 may be a chamber with transparent (or translucent) walls that definea cavity to hold a fixed volume of a fluid sample, such as a gas or aliquid. Alternatively, the sample holder 230 may form part of a channel(e.g., a microfluidic channel) through which a fluid such passes. Forinstance, the sample holder 230 may even be part of a human or animalbody, such as a blood vessel, and the sample may be a bodily fluid, suchas blood. In another embodiment, the sample holder 230 is a surface thatholds or supports adsorbed sample molecules. The sample holder 230 maydisposed microns (e.g., 1 μm, 5 μm, 10 μm, 25 μm, 50 μm, etc.) tomillimeters (e.g., 1 mm, 5 mm, 10 mm, 25 mm, 50 mm, etc.) to centimeters(e.g., 1 cm, 2.5 cm, 5 cm, 10 cm, etc.) from the lens array 220.

The parallel collimated beams 221 illuminate the sample 231, whichselectively reflects and/or absorbs some or all of the incidentradiation. In some instances, the sample holder 230 will be an existingstructure such as a human body, clothing, a piece of baggage, anpotential hazard, a pharmaceutical container, or an automobile partwhere the sample 231 is interrogated by the beams 221 and the absorbed,transmitted or backscattered light is detected. In such cases, it may bedesirable to include a telescope to direct the beams 221 toward thetarget and simultaneously to collect the backscatter, as shown in FIGS.6 and 7.

A detector 350 (FIG. 3) detects the reflected or transmitted portions ofthe parallel collimated beams 221 and produces an indication of thesample's spectrum. If desired, a cylindrical lens 340 (FIG. 3) can beused to circularize the collimated beams 221 transmitted through thesample holder before illuminating the detector 350. The detector 350generates an electrical signal (e.g., a photocurrent) that varies inamplitude as a function of the intensity of the detected radiation.

Compared to spectroscopy systems that use WBC, the spectroscopy system200 shown in FIG. 2 is simpler to build and align, more rugged, and morecompact. As discussed later, it also more immune to beam-steering thatis typical of diffraction based WBC as the lasers are tuned. Using smalllenses 222 to collect the light from the QCLs 212 in the laser array 210preserves the transverse spacing properties of the array, whilemaximizing collection efficiency into the far field 230.

Another embodiment of tunable laser arrays with microlenses is theanalysis of gases using multipass absorption cells, in which theentrance hole diameter is typically small in relation to the size of theentrance mirror, as shown in FIG. 4. In the particular embodiment inFIG. 4, a single collimated laser beam 421 is injected into a cavitydefined by two spherical mirrors 420. The beam 421 bounces back andforth many times before it exits the cell from an injection hole 430 inthe form of a beam 440 that can then be detected. Due to the extremelyclose spacing of adjacent lasers in a QCL array, many laser beams can becoupled into an identical cell, as shown in FIG. 5, which depicts theidentical optical cavity 420 of FIG. 4 but uses an array of fivesubstantially collimated, non-coincident laser beams 521 spaced 200microns apart. While the entrance hole 430 is small, it can accommodatethe substantially collimated collection of laser beams shown in FIG. 2,which is very useful in that more than one frequency can then beinjected to the same cavity. Note that in FIG. 5, all five beams exitfrom the same hole 430 as indicated by rays 540. (Rotation of the rays540 with respect to the fixed cell with each pass cause the rays 540 toappear coincident when viewed from the side in FIG. 5.)

Using multiple laser beams 521 increases the number of chemical speciesor aerosol that can be interrogated in a single apparatus and is enabledby the realization that array systems that use microlenses may producenon-overlapping beams, but that this is not at all detrimental to theperformance in certain cases. Also noteworthy is that unlike theanalysis situation depicted in FIGS. 2 and 3, where a spherical lens maystill be used for array collimation due to the short distances involved,using a single lens to collimate an array such as depicted in FIG. 1 isnot implementable in a multipass cell apparatus or standoff detectionsituation, where the beams traverse distances exceeding severalcentimeters.

Another embodiment involves the use of laser arrays with microlenses fora standoff detection such as that depicted in FIG. 6. Standoff detectionmay occur over a distance of centimeters (e.g., 1 cm, 10 cm, 25 cm, 50cm, etc.) to meters (e.g., 1 m, 10 m, 25 m, 50 m, 100 m) or evenkilomters (e.g., 1 km, 10 km, 25 km, 50 km, 100 km, etc.). Here, theoutput of a tunable laser array is substantially collimated usingmicrolenses 601 and coupled to a refractive telescope 602 beforepropagating into the far field. In this embodiment, it is thebackscattered light from a target 603 that is collected and analyzed.For example, light from a microlensed array of DFB QCLs is directedtoward a surface on which one or more chemicals are adsorbed. Atelescope directs the collimated beams from the array toward the targetand collects the backscattered photons at detector 604.

Similarly, a reflective telescope could be used for standoff detection,as in FIG. 7. Here, the output of a tunable laser array is collimatedusing microlenses 701 and coupled to a reflective telescope 702 beforepropagating into the far field. Backscattered light from a target 703that is collected by the telescope and directed onto a detector 704.

In such standoff detection embodiments, light from the array impinges onthe adsorbed material, is differentially absorbed based on the materialswavelength-dependent molecular fingerprint, and scatters in manydirections. Some of the light is scattered back toward a telescope whichis used to collect a portion of the backscattered radiation and directit onto the detector. In this embodiment, the laser spacing itselfbecomes insignificant relative to the typical beam divergence as thelaser beams are propagated into the far field. For instance, in thestandoff embodiments described above, the beam divergence of each offive laser beams after lens collimation may be about 0.5 degrees toabout 8.0 degrees (e.g., 1.0 degree, 2.0 degrees, 3.0 degrees, 4.0degrees, 5.0 degrees, 6.0 degrees, or 7.0 degrees). Depending on thisdivergence, at propagation distances of several meters each beam'sintensity becomes distributed over many times the beam diameter. Thetiny lateral spacing between lasers becomes vanishingly small.

This is illustrated in FIG. 8, where the raytrace of five tunable lasersspaced 200 microns apart is executed into the far field (without firstcoupling to a telescope). From left to right, the distance between thecollimated array and the target increases. The uniformity of the overallphoton flux increases as distance from the array increases, such that atlong distances it is no longer evident that the light can be traced backto spatially separated individual emitters. While a telescope may stillbe employed, in particular to collect diffuse backscattered light from atarget, it is not always required for directing the array output towardthe target.

Another embodiment utilizes a similar experimental setup as shown inFIG. 6 and FIG. 7 and as described above, but instead uses the solidscattering surface for reflection only. In such an embodiment, it is theinterstitial air that is being sampled for the presence of a gas oraerosol of interest. One example is the examination of vaporizedexplosives residues, rather than the bulk explosive itself.

It is noteworthy that in the case of backscatter collection in standoffembodiments, light from each laser is scattered from the surface over awide range of angles. This leads to a divergence in the back reflectedlight that tends to dominate the SNR limitations of the experiment farmore than the imperfect overlap of the beams from the laser array. For agiven detector position with respect to a scattering target distance raway, the collected backscattered photon flux has a 1/r² dependence.

Note also that in the case of far-field propagation, illuminating atarget with non-coincident laser beams is not only simpler, but actuallymay be better than in a situation where WBC is implemented. The reasonhas to do with the slight pointing error that is introduced in an openloop WBC implementation as the laser is tuned based on the gratingequation. Specifically, in open-loop WBC, the angle at which the gratingshould be placed relative to the array or transform lens such that thebeams co-propagate may be deduced from the following grating equation:d(sin θ_(m)+sin θ_(n))=mλ _(n)  (2)In Equation (2) d is the groove spacing of the grating, θ_(m) is theoutput angle of the m-th diffraction order, θ_(n) is the incident angleof the n-th laser beam on the grating, and θ_(n) is the wavelength ofthat (n-th) laser. The incident angles, θ_(n), of the laser elements inthe array are all different and satisfy the following equation:

$\begin{matrix}{\theta_{n} = {\theta_{grating} + {\tan^{- 1}\left( \frac{x_{n}}{f} \right)}}} & (3)\end{matrix}$

In Equation (3) x_(n) is the position of the n-th laser in the array andƒ is the focal length of the transform lens. For all the beams toco-propagate, all the lasers in the array have the same output angleθ_(m). However, as a laser is tuned in wavelength using injectioncurrent or heatsink. temperature, the resulting change in λ_(n) is oftengreat enough that the resulting shift in angle θ_(m) corresponds to amisdirected laser, an error which is amplified for long distancepropagation. By using a lens array and perhaps one or more reflective orrefractive optics, no appreciable change in far-field pointing isintroduced for the 10 cm⁻¹ per-laser tuning in λ_(n) that is typicallyinduced with laser current and/or heatsink tuning. This is alsobeneficial for applications where high power has to be transmitted overlarge distances such as infrared countermeasures and standoff detection.

Another possible embodiment of laser arrays coupled to lens arrayswithout WBC includes cavity enhanced spectroscopy, including asIntegrated Cavity Output Spectroscopy or Cavity Ringdown Spectroscopy.In these systems, light is injected into a cavity composed of two ormore highly reflective surfaces such that it traverses a pathlength thatis many times the mirror separation itself. Whether introduced viaeither an axial or “off-axis” optical injection geometry, the concavityof the cavity mirrors serves to continually focus and direct theinjected radiation. In principle, many parallel laser beams can becoupled into a single cavity, increasing the number of analytes that canbe studied while not increasing instrument size, weight, or powerappreciably. Here again, actually overlapping the laser beams isgenerally not necessary, as it is the residence time of each frequencywithin the cavity that serves to define the interaction length, ratherthan the precision of the alignment. This residence time can determinedindividually for each array member by measuring the decay time of thatparticular frequency from the optical cavity. For an empty cavity, thisdecay time, τ, is approximated by:

$\begin{matrix}{\tau = \frac{L}{c\left( {1 - R} \right)}} & (4)\end{matrix}$Where τ is the average cavity residence time of a photon, L is theseparation of the mirrors, c is the speed of light and R is thereflectivity of the cavity mirrors. For example, in a two-mirror cavityformed by mirrors with identical 99.95% reflectivity spaced 1 m apart,the time constant is 6.67 μs.

Another novel embodiment of laser arrays coupled to lens arrays is acompact sensor for examining fluids in the human body, as illustrated inFIG. 9. Here, the output of a laser array 901 includes at least onelaser that propagates through a matched microlens array 902 and shineson the skin of a human finger 903 which has been placed on a flatsurface 904. Some of the light penetrates the outer skin layers andinteracts with body fluids, such as blood. A portion of the lightcarrying the absorption signature of the analyte of interest is thenreflected 905 from the finger and detected by a detector 906. In somecases, the detector also includes a focusing lens for collecting thereflected light onto the detector surface. In this system, it becomespossible to analyze, for example, blood glucose levels in humans withoutWBC of the tunable laser array. Such a sensor could also be constructedin such a way that the laser energy is directed through a layer of skindirectly onto a detector. For example, there exist thin sections of thehuman body through which infrared light may transmit. In such a case,the transmitted light would carry the absorption spectrum of interest.

Advances in lithographic fabrication techniques have made it possible tofabricate microlenses 222 with a variety of surface topographies,including astigmatic and aspheric shapes, allowing for even betterperformance than may be obtained using, for instance, typicalmicrolenses 222 with a spherical surface figure. This is extremelyadvantageous in the case of QCL collimation, due to the fact that theselasers exhibit rapid and non-uniform divergence in the X and Ydirections (Where Z is the direction of beam propagation into the farfield). Aspheric lenses are customary for the collimation of laserdiodes and QCLs using individual optics, but have to our knowledge notbeen expanded to the collimation of QCL arrays 210.

Further, due to the nature of the lithographic process of lens arrayfabrication, it is straightforward to fabricate standalone microlenses222 and microlens arrays 220 where each lens is non-uniform in its X andY physical dimensions in order to provide a more full collection oflight emitting from the QCLs, which have sharply higher divergence inthe direction perpendicular to the plane of the substrate (Y) than inthe direction that is parallel to the laser substrate (X). FIG. 10 showsa contour model of a 15-element microlens array 220 that illustrates howa lens array 220 can be fabricated so as to maximize the collectionefficiency of QCL array 210 (FIG. 2) in which divergence is higher inthe direction perpendicular to the substrate than in the plane of thesubstrate (also the plane of array propagation). In FIG. 10 the width ofan individual microlens 222 is 75 microns while the height is twicethat, 150 microns. Such a microlens array 220 will have highercollection efficiency for a QCL array 210 where the spacing betweenemitters is about 75 microns. Lens efficiency can also be improved byadding some aspheric character to the lens surface, so as to reducespherical aberration.

Once packaged, the QCL array 210 with lens array 220 may take the formof the model shown in FIG. 11 where it may serve as the light source fora broader spectroscopic system. Here the laser array 210 is placed on anAluminum Nitride submount 1110 which in turn rests on a Thermoelectriccooler 1120 that serves to maintain the temperature of the QCL array210. Heat from the QCL array 210 is transferred to the Copper/Tungstenmetal block (heatsink) 1130. As each QCL 212 in the array 210 isindividually addressable, the submount 1110 may be patterned with metaltraces that connect the lasers 212 in the array 210 with the connector1150 through the package frame 1140. One aspect of the design shown inFIG. 11 is that the QCL array 210 and the lens array 220 are placed onthe same height stack such that small changes the expansion andcontraction of the metal block 1130, Kovar frame 1140, and/or the cooler1120 do not result in changes in alignment of the lens array 220 withrespect to the laser array 210.

Other possible embodiments of laser arrays coupled to lens arrayswithout WBC include infrared countermeasures (IRCM), detection ofexplosives or chemical warfare agents, monitoring of toxic industrialcompounds, free-space optical communication, and the marking and cuttingof plastics and metals.

While various inventive embodiments have been described and illustratedherein, those of ordinary skill in the art will readily envision avariety of other means and/or structures for performing the functionand/or obtaining the results and/or one or more of the advantagesdescribed herein, and each of such variations and/or modifications isdeemed to be within the scope of the inventive embodiments describedherein. More generally, those skilled in the art will readily appreciatethat all parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the inventive teachingsis/are used. Those skilled in the art will recognize, or be able toascertain using no more than routine experimentation, many equivalentsto the specific inventive embodiments described herein. It is,therefore, to be understood that the foregoing embodiments are presentedby way of example only and that, within the scope of the appended claimsand equivalents thereto, inventive embodiments may be practicedotherwise than as specifically described and claimed. Inventiveembodiments of the present disclosure are directed to each individualfeature, system, article, material, kit, and/or method described herein.In addition, any combination of two or more such features, systems,articles, materials, kits, and/or methods, if such features, systems,articles, materials, kits, and/or methods are not mutually inconsistent,is included within the inventive scope of the present disclosure.

Also, various inventive concepts may be embodied as one or more methods,of which an example has been provided. The acts performed as part of themethod may be ordered in any suitable way. Accordingly, embodiments maybe constructed in which acts are performed in an order different thanillustrated, which may include performing some acts simultaneously, eventhough shown as sequential acts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in the United States Patent Office Manual ofPatent Examining Procedures, Section 2111.03.

What is claimed is:
 1. A system for sensing a spectroscopic signature ofa sample, the system comprising: an array of quantum cascade lasers(QCLs) to emit an array of substantially parallel laser beams comprisinglasers beams at different wavelengths; an array of collimating lenses,in optical communication with the array of QCLs, to substantiallycollimate the array of parallel laser beams; and a multipass absorptioncell, in optical communication with the array of collimating lenses, toreceive the array of parallel laser beams from the array of collimatinglenses, to direct the array of parallel laser beams along multiplepasses through a least a portion of the sample, and to emit the array ofparallel laser beams after transmission through the at least a portionof the sample; and a detector, in optical communication with themultipass absorption cell, to detect at least a portion of the array ofparallel laser beams so as to provide a signal representative of thespectroscopic signature of the sample.
 2. The system of claim 1, whereinthe sample comprises at least one of a bodily fluid, a gas, an adsorbedsolid, an adsorbed liquid, a gas phase component of a surface-adsorbedsolid, and a gas phase component of a surface-adsorbed liquid.
 3. Thesystem of claim 1, wherein optical path length between the array ofcollimating lenses and the sample is at least about 10 cm.
 4. The systemof claim 1, wherein the array of QCLs is configured to tune a wavelengthof at least one laser beam in the array of substantially parallel laserbeams without changing a far-field pointing angle of the at least onelaser beam.
 5. The system of claim 1, wherein the array of QCLs isconfigured to generate each laser beam in the array of substantiallyparallel laser beams at a different wavelength.
 6. The system of claim1, wherein the array of collimating lenses includes at least one lenshaving a beam divergence angle of about 0.5 degrees to about 5.0degrees.
 7. The system of claim 1, wherein the array of collimatinglenses includes at least one of an aspheric lens, a spherical lens, adiffractive lens, and a graded-index lens.
 8. The system of claim 1,further comprising: a telescope, in optical communication with the arrayof collimating lenses, to cause the array of substantially parallellaser beams to propagate towards the sample.
 9. The system of claim 8,wherein the telescope is further configured to collect radiationscattered from the sample.
 10. The system of claim 1, furthercomprising: at least one of a current source in electrical communicationwith the array of QCLs and a a heatsink in thermal communication withthe array of QCLs to tune a wavelength of at least one laser beam in thearray of laser beams without changing a far-field pointing angle of theat least one laser beam.
 11. A method of sensing a spectroscopicsignature of a sample, the method comprising: (A) emitting an array ofspatially non-coincident laser beams from an array of quantum cascadelasers (QCLs); (B) substantially collimating the array of spatiallynon-coincident laser beams emitted in (A); (C) illuminating the samplewith the array of spatially non-coincident laser beams collimated in (B)so as to produce radiation that is scattered, reflected, and/ortransmitted by the sample; (D) detecting at least a portion of theradiation that is scattered, reflected, and/or transmitted by the samplein (C); and (E) providing a signal representative of the spectroscopicsignature of the sample based on the at least a portion of the radiationdetected in (D).
 12. The method of claim 11, wherein (A) comprisesgenerating each laser beam in the array of spatially non-coincidentlaser beams at a different wavelength.
 13. The method of claim 11,wherein (B) comprises transmitting the array of spatially non-coincidentlaser beams through at least one of an aspheric lens, a spherical lens,a diffractive lens, and a graded-index lens.
 14. The method of claim 11,wherein (B) comprises transmitting the array of spatially non-coincidentlaser beams through at least one lens having a beam divergence angle ofabout 0.5 degrees to about 5.0 degrees.
 15. The method of claim 11,wherein (C) comprises transmitting the array of spatially non-coincidentlaser beams towards at least one of a bodily fluid, a gas, an adsorbedsolid, an adsorbed liquid, a gas phase component of a surface-adsorbedsolid, and a gas phase component of a surface-adsorbed liquid.
 16. Themethod of claim 11, wherein (C) comprises transmitting at least one ofparallel laser beams and intersecting laser beams.
 17. The method ofclaim 11, wherein (C) comprises transmitting the array of spatiallynon-coincident laser beams to the sample over an optical path length ofat least about 10 cm.
 18. The method of claim 11, wherein (C) comprisestransmitting the array of spatially non-coincident laser beams to thesample via a telescope so as to cause the array of spatiallynon-coincident laser beams to converge when propagating towards thesample.
 19. The method of claim 11, further comprising: (F) tuning awavelength of at least one laser beam in the array of spatiallynon-coincident laser beams without changing a far-field pointing angleof the at least one laser beam.
 20. The method of claim 11, furthercomprising: (G) confining at least a portion of the sample in opticalcommunication with the QCLs such that the array of spatiallynon-coincident laser beams intersects the at least a portion of thesample.
 21. The method of claim 20, wherein (G) comprises confining theat least a portion sample with at least one of a transmissive cell, anadsorbing surface, a micro fluidic channel, and a multipass absorptioncell.
 22. The method of claim 11, further comprising: (H) flowing atleast a portion of the sample with respect to the array of spatiallynon-coincident laser beams.